Strut spacers are used when lowering the front of the car. They sit between the steering arm and strut to preserve the original roll steer. By altering the angle of the lower control arm they will potentially remove unwanted bump steer. Also known as Roll Center Adjusters.
Overview
When the struts are lowered significantly over standard, the suspension arms are close to parallel with the ground and the tie rods are inclined up instead of down. They don't have the correct steering geometry so Roll Center Adjusters space out the steering arms.
These offset kind will also widen your track by moving the strut outward (you could alternatively reverse them to move the strut inward):